Automatic volume control



May 15, 1951 T. A. READ ETAL AUTOMATIC VOLUME CONTROL 2 Sheets-Sheet l Filed Feb. 19, 1947 -RE/xo,

F115 FELD,

May 15, 1951 T. A. READ E'rAl.4 2,552,586

AUTOMATIC VOLUME CONTROL Filed Feb. 19, 1947 2 sheets-sheet 2 l :E I E- EJ.. G12/D BIAS VOLTAGE [u "l D2 Q b 0 -2 E O 1 0/m L/NE PLATE vom-AGE GM, F1 E- .El 3o c lo -2 -4 -s -a -fo -lz |4 we la -2o com-.QOL een: ems /N vou-.s @We/Mm THDMA5A READ,

HERBERT I FLISFELQ,

UNITED STATES PATENT OFFICE AUTOMATIC VOLUME CGNTROL Thomas A. Read and Herbert I. Fusfeld, Philadelphia, Pa.

Application February 19, 1947, Serial No. 729,639

2 Claims. (CL 179-1'71) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757) 1 The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

In the drawings:

Fig. 1 is an electric circuit diagram.

Fig. 2 is a graphic representation of characteristic curves showing the plate voltage-plate cur- This invention relates to a control circuit for 5 rent of a pentcde voltage amplier tube. maintaining an electric signal at a constant Fig. 3 isagraphic representation in which voltstrength. age gain of a voltage amplier tube is plotted This invention has wide applications since it against control grid voltage of the same tube. may be applied to apparatus for testing metal by i That embodiment of the invention selected for sonic methods, or as an automatic volume conillustration in the accompanying circuit diagram trol in radio communication receivers and in (Fig. 1) is made up of an input terminal 5 conother vacuum tube amplifying circuits, the gain nected to the control grid 6 of a voltage amplifier of which it is desired to vary in accordance with tube 7. Tube l may be a tube identified by Radio the strength of some signal. y Corporation of America Type No. 6K7. Tube 'l is One object of this invention is to provide a shown as being a pentode which is a triple-grid, gain-control circuit in which the control grid of super-control amplifier of the heater-cathode an electronic tube is especially arranged to opertype. It is recommended for service in the radioate upon decreases in rather than increases in frequency (R-F) or intermediate-frequency (I-F) superimposed negative bias. stages of radio receivers. It has the ability to It is another object of this invention to prohandle unusual signal voltages without crossvide, in an amplifying circuit having a voltage modulation and modulation-distortion so that it amplifying tube, a diode rectifier tube whose is adaptable to the R-F and I-F stages of radio cathode is connected to the plate of the voltage receivers employing automatic volume control ampliiier tube and whose plate is normally mainn (AVC). Tube l has a plate 8, a suppressor grid tained at a suiiicient negative voltage so that '"5 9, a screen grid I 0, and a cathode II. passage of a direct current through the diode Cathode II is connected to ground I2. Screen rectier tube causes the control grid of the voltgrid I0 is connected to the positive pole of the age amplier tube to be less negative than the B battery or other source of direct current. To negative bias at which it is normally maintained. maintain the voltage of screen grid Iii constant It is a further object of this invention to prowhile the voltage of control grid 6 is varied, a vide, in an electric control circuit having a voltvoltage regulator tube I3 is connected between age amplifier tube and a diode rectier tube and the ground I2 of cathode II and the screen grid in which the operating plate voltage of the volt- I0. age amplier tube is maintained at a relatively A load resistor I4 is connected between plate 8 low value, means for maintaining the screen grid and the positive terminal of the B battery or voltage of the voltage amplifier tube at a relativeother source of direct current. A second resistor ly high value which remains constant as the con'- I5 is connected to plate 8 and is connected at its trol grid voltage is changed. opposite end to an AVC resistor I6 in series with It is a still further object of this invention to Ya battery I'I connected at its opposite end to provide a method of stabilizing a voltage in which 40 ground I2. Battery I8 is connected in opposite the negative voltage bias applied to the control or bucking relation to battery I'I so that the posgrid of a voltage amplifier tube is varied in ac- `itive pole of battery I8 is also connected to ground cordance with the variations in a voltage which is I2. The negative pole of battery I8 is connected to be stabilized and within a range below that to the plate I9 of a diode rectier tube 2!! whose value of the negative voltage bias at which maxcathode 2| is connected to plate 8 through a imum voltage amplication is secured in said blocking condenser 22 which forms part of the voltage amplifier tube. output terminal connection 23.

These and other objects of this invention will An AVC lter is connected at one side between be readily apparent to those skilled in the art, resistors I5 and I6. This AVC filter comprises a or will be made known from the accompanying condenser 24 and a resistor 25 connected by means drawings and specication to which reference is of the input terminal 5 to control grid 5. here made for a further exposition of this in- Fig. 2 shows schematically the plate vb1tagevention and at the end whereof the specific ieaplate current characteristic curves for a typical tures which it is desired to protect by Letters Patpentode voltage amplier tube such as tube l. ent are claimed. The load line L is drawn corresponding to the value of the plate load resistor I4 and of the plate supply voltage. The intersection of this load line with a characteristic curve gives the value of the plate Voltage which corresponds to any given grid voltage. It will be noted that these points ofY intersection are much closer together for small grid bias voltages than for large grid bias voltages because of the greatly increased slope of the current vs. Voltage curves at low plate voltages, that is to say, in Fig. 2, the distance VW or WX is much less than the distance XY'o'r YZ, all these distances being measuredy along the -l'oad` line L.

The advantage of this invention over the oonventional AVC circuit is shown in`Fig; 3'. The' curve C in Fig. 3 plots the data obtainedl as the result of a dynamic test of a 6K7 or a SSK'Z tube using a plate resistor of 50,000 ohms, -a plate supply voltage of 350 volts, and a screen grid voltage of 118 Volts. In this figure, the Voltage gain is plotted as a function of the control grid voltage. In the conventional AVC circuit, the. control grid biasz potential isl always numerically greater than the value at which the gain'is maximum. A reduction in gain is effected by increasing the bias. In this new circuit, the control grid bias voltage is always less than the value `at which the gain is maximum. A reduction of gain is eifected by decreasing the bias.

From Fig..3 it is apparent that a much more rapid change in gain, as the grid bias is changed, is achieved by the new circuit than by those heretofore used; Y

The operation of thatmodication of the device shown in Fig. 1 is as follow. The signal to be amplified iszapplied through the input connection 5 to theV control grid Ci of the voltage amplifier tubev 1. The amplified signal appears across resistor Il! and is applied to the electrodes I9 andZI of diode rectiiier tube 2-0. In the absence of a signal and at a steady state of current, a constant voltage equal to the diierence between thervoltages` of batteries I'I and I8 appears between the` plateY I9 and` theV cathode 2I ofthe diode rectifier tube 20. The plate I9 is nega-tive with respect to the cathode 2 I. Likewise control grid? 6 of voltage amplifier tube 'I is negative with respect to ground. If the magnitude `of the amplified signal from the tube 'I is increased' until the peak value of the alternating voltage appearing across resistor I5 exceeds the voltage difference ofi battery I8 minus battery I1, current will flow through the diode rectifier tube '20.. This current causes a voltage drop across resistor I6. This voltage drop makes the control grid B 'of tube 1 less negative and thusdecreases the amplification factor of tube 'I in the manner described below.

This improvedl method of AVC operates by shifting the operating point of the tube from a plate voltage at which the characteristic curves (as seen in Fig. 2) have a small slope toa lower plate voltage at which theslope is much greater. In this region of increased slope the amplificationfactor of .the tube is :greatly reduced:

Itis important ltothe functioning of this circuit that the voltage of screenY grid I0 ybe held con- 4 stant as the voltage of control grid 6 is changed. One way of accomplishing this is shown in Fig. lY wherein the voltage regulator tube I3 is shown connected between the screen grid I0, and the ground 'I2 connected to cathode II.

In general terms, thisr improved method of AVC operates through using an increase in signal level to decrease the bias on the control grid 6. The plate voltage supply circuit has a high resistance (I4) whereas the screen grid voltage supplyhas a very low resistance.

We claim:

1. In an electric circuit for controlling the amplification applied to an electric signal passing through said' circuit, a vacuum tube having at least five electrodes consisting of a cathode, control grid, screen grid, suppressor grid, and plate for` connection to a load, a supply voltage connected to the plate of said tube, a separate voltage supply connected to the screen grid of saidtubel forv maintaining it at a higher constant voltage than said plate, a biasing voltage source applied to thev control grid and of such magnitude that said tube operates below the knee of the characteristicV gain, control grid bias curve, a condenser in series between plate and the load, a rectifier connected between the load end of the condenser andcathode, and a circuit connected across the rectifier and to the control grid for applying a voltage of opposite polarity to the grid bias voltage with an increase of signal applied to the control grid.

2. In arr electric circuit for controllingthe amplification applied to an electric signal passingV through said circuit, a vacuum tube having at least five electrodes consisting of a cathode, control` grid, screen grid, suppressor grid and plate, a supply voltage connected to the plate of said tube, a separate voltage supply connected to the screen grid of said tube for maintaining -it at a higher constant voltage than said plate, a negative biasing voltage source applied to the control grid and of such magnitudeI that said tube operates to maintain the gai-nv of the tube at a high level, a rectier in the plate-cathode circuit of the tube, and a circuit connected between the output of the rectifier` and the control grid for applying a positive voltage to said grid to `reduce the gain with an increasecf signal applied to the control grid.

'THOMAS A. READ. HERBERT I. FUSFELD.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,959,010 Tellegen 'May 15, 1934 1,985,352 Numans Dec. 25, 1934 2,010,253 Barbon Aug. 6, 1935 2,083,626- Wirin June 15, 1937 2,120,995 Vande Riet June 21, 1938 2,175,990- I-Iirsch Oct. 10, 1939 2,259,873

Baumgartner Oct. 21, 194-1 

